The development of new technologies has been always accompanied by the access to functional materials with targeted and exceptional properties. Among these materials and looking towards the future, layered materials and metal halide perovskites outline a prospective path for their potential application in optoelectronic, spintronic and quantum technologies.[1–4] However, for their successful integration into devices and the development of new applications, it is key to understand the relationship between composition, crystal structure and optical/magnetic properties and how to control them.

Layered hybrid organic-inorganic metal halide perovskites (HOIPs) have emerged as promising materials for optoelectronic and spintronic applications, namely due to their tunable bandgap, high carrier mobility, strong spin-orbit coupling and magnetic ordering.[1,2] Indeed, HOIPs are an ideal platform for optical (photons) and magnetic (spins) tunability due to their chemical and structural versatility.[5,6] In this line, two case studies are presented. The first one is focused on modulating the optical properties, concretely the photoluminescence (PL), by strain engineering. We report the tuning of the micro-PL emission of 2D lead-bromide HOIP flakes subject to biaxial strain. To generate the mechanical strain, we placed the flakes by viscoelastic stamping on a rigid SiO₂ ring platform, leading to the formation of domes. At low temperatures, we found that a strain < 1% can change the PL emission spectrum from a single peak (unstrained) to three well-resolved peaks. Combining temperature-dependent micro-PL and Raman spectroscopy mapping and reverse mechanical engineering strain modeling, we confirm that the emergence of the two new PL peaks is related to tensile and compressive thermo-mechanically generated strain coexisting along the flake surface and thickness.[7] Our findings provide new insight into strain-based optoelectronic and sensing devices using 2D HOIPs, leveraging on the material composition selection and substrate platform design. The second case deals with the control of the magnetic properties by varying the transition metal (Cu²⁺, Mn²⁺ and Co²⁺), organic spacer (alkyl- and aryl-ammonium) and perovskite phase (Ruddlesden-Popper and Dion-Jacobson). We show that for Cu²⁺ HOIPs, an increase of in-plane anisotropy and a reduction of the interlayer distance lead to a change in their magnetic behavior from a 2D ferromagnet to a quasi-3D antiferromagnet. In contrast, the magnetism of Mn²⁺ HOIPs is intrinsically characterized by antiferromagnetic intralayer interactions. Finally, Co²⁺ crystals with a non-perovskite structure present a dominant paramagnetic behavior. Therefore, our results demonstrate that the chemical flexibility of HOIPs can be exploited to develop novel layered magnetic materials with tailored magnetic properties.[8]